By Richard A. Lovett

Global warming may be making pesticide residues, heavy metals and household chemicals more dangerous to fish, wildlife and, ultimately, humans, scientists warn.

At the North American branch of the Society of Environmental Toxicology and Chemistry's 31st annual meeting in Portland, Oregon, on 8 November, environmental chemists warned that complex interactions between chemistry and climate change might be making chemicals more toxic and the environment more susceptible to damage.

For example, Erin Mann, a graduate student studying environmental chemistry at the University of Toronto in Scarborough, Canada, said that melting sea ice in the Arctic Ocean exposes more seawater to the atmosphere, which may make it easier for toxic chemicals in arctic waters to escape into the air. "So global warming could produce more air pollution in the arctic," she said.

Nor are only distant climes likely to be affected. Theodore Valenti, an aquatic toxicologist who works at the US National Research Council but is on assignment to the federal Environmental Protection Agency in Duluth, Minnesota, noted that climate change will cause differences in the movement, quality and distribution of water that could affect stream acidity all over the world. This would alter the toxicity of chemicals such as pharmaceuticals, which make their way into these streams when they are excreted into waster water and flushed down the toilet.

Drugs are designed so that small changes in acidity alter their bioavailability, helping to route them to the bodily tissues where they are needed. But when they reach the environment, says Valenti, "it's the same thing. I've seen upwards of 10- to 20-fold differences in toxicity at pH 9 compared with pH 6".

Run-off repercussions

Heavy-metal toxicity could also be affected by climate change, said Marjorie Brooks, an aquatic toxicologist at Southern Illinois University in Carbondale.

For copper, she said, the LC50 (the concentration lethal to half of any given species over a certain time) decreases with temperature -- meaning that toxicity increases. The effect is so strong, she said, that if Earth continues to warm at the current rate, the LC50 for one species she has studied, fathead minnows (Pimephales promelas), will be only half as much in 2060 as it is now. "This has some pretty severe implications for water-quality criteria," she said.

And these are just the relatively direct effects of climate change.

Will Clements, an ecotoxicologist at Colorado State University in Fort Collins, noted that warming in the Rocky Mountains is expected to reduce the mass of lying snow, thus decreasing summer run-off. This, in turn, will lower the depth and turbidity of water in mountain streams, increasing the amount of damaging ultraviolet light that reaches the stream beds.

To investigate this effect, Clements and his team placed screens over small segments of a dozen mountain streams to block ultraviolet light, and left them there for 60 days. At the end of that time, the team found higher populations of stoneflies, mayflies and caddis flies in the screened segments than in adjacent unscreened ones.

The conclusion: low stream flows might harm aquatic organisms by increasing their exposure to ultraviolet light.

Rethinking experiments

Climate change is also set to challenge scientists' methods for studying toxicity.

Nicholas Gard, an ecologist at science and engineering consulting firm Exponent in Bellvue, Washington, said that an entire smorgasbord of factors must be taken into account when considering toxicity. For example, global warming could cause toxic chemicals to become more volatile, and so more prone to dispersing widely in the wind. Or efforts to offset warming by injecting carbon dioxide into underground rocks could make aquifers more acidic, increasing their ability to leach dangerous metals out of rocks, and leading to higher concentrations of these metals in springs and rivers. "All of these are things we need to think about," he said.

Jennifer Goss, a toxicology graduate student with the US Army Corps of Engineers in Vicksburg, Mississippi, studies the effects of climate change on lead toxicity -- a topic that is of interest to the military because lead can leach from spent bullets on shooting ranges into water.

Goss studied Daphnia magna, a tiny freshwater crustacean used in many aquatic toxicity studies. "We saw greater sensitivity to lead at higher temperatures," she said.

This is important not only because it means that global warming could alter the ecotoxicity of lead, but also because Daphnia experiments are customarily done at a single temperature in each study. This raises the question of whether conventional experimental protocols will be relevant in a warmer world.

"We have to have a fundamental change in the way we approach assessing risk of chemicals in the environment," says Ralph Stahl, an environmental toxicologist in the remediation department of the Dupont Corporation, a chemical company based in Wilmington, Delaware. "The conditions under which we test haven't changed in 30 years."